The technical field of this invention is dynamoelectric machines with housings that include liquid cooling passages. Some dynamoelectric machines, such as traction motors for electric vehicles and high capacity generators for hybrid vehicles, generate high heat in a small package; and the thermal efficiency of heat removal is an important part of their design. Liquid coolant flowing through internal passages in the housing provides an efficient method of removing heat from the machine.
For an electric or hybrid vehicle motor or generator application, an additional goal of the design is a low pressure drop in the liquid coolant circulation system, since the coolant circulating pump is a power drain in vehicle operation and the circulating pressure drop can thus have a significant effect on vehicle range. One way of decreasing the circulating pressure drop is the use of a "single loop" design, where a single coolant passage, extending in axial width across a substantial portion of the stator, conducts liquid coolant circumferentially once around the outside of the stator. The shorter coolant passage, as opposed to a coiled passage of many turns, results in a lower pressure drop, especially if it is somewhat larger in cross-sectional area; but it also may result in a decrease in heat transfer efficiency in the coolant passage, due to the more limited contact of the coolant with the housing and a decrease in velocity for the same volume liquid flow rate in a larger volume passage. In addition, unless a smooth transition is made between the inlet and outlet of the single coolant passage to the liquid coolant supply conduits, substantial pressure drops may occur at the inlet and outlet.